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1 Astro Lecture 5 Eclipses of the Moon and the Sun, and other odd events Lunar eclipse Solar eclipse 1

2 Brief Review Why do we see phases of the Moon? What causes eclipses? 2

3 Why do we see phases of the Moon? Lunar phases are a consequence of the Moon s 27.3-day orbit around Earth. 3

4 Phases of Moon Half of the Moon is illuminated by the Sun and half is dark. We see a changing combination of the bright and dark faces as the Moon orbits Earth. How to Simulate Lunar Phases 4

5 Phases of the Moon Phases of the Moon 5

6 Moon Rise/Set by Phase Time the Moon Rises and Sets for Different Phases 6

7 Phases of the Moon: 29.5-day cycle new crescent first quarter gibbous full gibbous last quarter crescent } } waxing Moon visible in afternoon/evening Gets fuller and rises later each day waning Moon visible in late night/morning Gets less illuminated and sets later each day 7

8 Thought Question It s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? A. First quarter B. Waxing gibbous C. Third quarter D. Half moon 8

9 Thought Question It s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? A. First quarter B. Waxing gibbous C. Third quarter D. Half moon 9

10 We see only one side of the Moon Synchronous rotation: The Moon rotates exactly once with each orbit. This is why only one side is visible from Earth. 10

11 What causes eclipses? The Earth and Moon cast shadows. When either passes through the other s shadow, we have an eclipse. 11

12 Lunar Eclipse Lunar Eclipse 12

13 When can eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. 13

14 Solar Eclipse Evolution of a Total Solar Eclipse 14

15 When can solar eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular. 15

16 Why don t we have an eclipse at every new and full moon? The Moon s orbit is tilted 5 to ecliptic plane. So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. 16

17 Summary: Two conditions must be met to have an eclipse: 1. It must be a full moon (for a lunar eclipse) or a new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). 17

19 What have we learned? Why do we see phases of the Moon? Half the Moon is lit by the Sun; half is in shadow, and its appearance to us is determined by the relative positions of Sun, Moon, and Earth. What causes eclipses? Lunar eclipse: Earth s shadow on the Moon Solar eclipse: Moon s shadow on Earth Tilt of Moon s orbit means eclipses occur during two periods each year 19

20 2.4 The Ancient Mystery of the Planets What was once so mysterious about the movement of planets in our sky? Why did the ancient Greeks reject the real explanation for planetary motion? 20

21 Planets Known in Ancient Times Mercury difficult to see; always close to Sun in sky Venus very bright when visible; morning or evening star Mars noticeably red Jupiter very bright Saturn moderately bright 21

22 What was once so mysterious about the movement of planets in our sky? Planets usually move slightly eastward from night to night relative to the stars. But, sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion. 22

23 We see apparent retrograde motion when we pass by a planet in its orbit. Mars Retrograde Motion 23

24 Explaining Apparent Retrograde Motion Easy for us to explain: this occurs when we lap another planet (or when Mercury or Venus laps us). But it is very difficult to explain if you think that Earth is the center of the universe! 24

25 Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor. 25

26 The Greeks knew that the lack of observable parallax could mean one of two things: 1. Stars are so far away that stellar parallax is too small to notice with the naked eye. 2. Earth does not orbit Sun; it is the center of the universe. With rare exceptions, such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away Thus the stage was set for the long, historical showdown between Earth-centered and Sun-centered systems. 26

27 What have we learned? What was so mysterious about planetary motion in our sky? Like the Sun and Moon, planets usually drift eastward relative to the stars from night to night; but sometimes, for a few weeks or few months, a planet turns westward in its apparent retrograde motion. Why did the ancient Greeks reject the real explanation for planetary motion? Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable. 27

28 In class exercises New Planet Suppose we discover a new planet in another solar system that has a circular orbit and an axis tilt of 35º. Would you expect this planet to have seasons? If so, would you expect them to be more extreme than the seasons on Earth? If not, why not? 28

29 View from the Moon Suppose you lived on the Moon, in which case you would see Earth going through phases in your sky. Assume you live near the center of the face that looks toward Earth. a. Suppose you see a full Earth in your sky. What phase of the Moon would people on Earth see? Explain b. Suppose people on Earth see a full Moon. What phase would you see for Earth? Explain c. Suppose people on Earth see a waxing gibbous Moon. What phase would you see for Earth? Explain d. Suppose people on Earth are viewing a total lunar eclipse. What would you see from your home on the Moon? Explain 29

30 A Farther Moon Suppose the distance to the Moon were twice its actual value. Would it still be possible to have a total solar eclipse? Why or why not? 30

31 A Smaller Earth Suppose Earth were smaller. Would solar eclipses be any different? If so, how? Would it still be possible to have a total solar eclipse? Why or why not? 31

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